CODEWITHSUNDEEP
multithreadingopenmp
knezi
knezi

Reputation: 257

Why does openMP hang on barrier within a while cycle?

I've got the following code:

    int working_threads=1;
#pragma omp parallel
    {   
        int my_num=omp_get_thread_num()+1;
        int idle=false;
        while(working_threads>0) {
            if(my_num==1)
                working_threads=0;
#pragma omp barrier                                                                                                                       
        }   
    }

If I run it, it every now and then hangs on the barrier. The more threads, the more likely this is to happen. I've tried to debug it with printf and it seems that sometimes not all threads are executed and thus the barrier waits for them forever. This happens in the first iteration, the second one is obviously never run.

Is it an invalid piece of code? If so, how can I change it? I need to run a while loop in parallel. It is not known how many loops will be executed before, but it is guaranteed that all threads will have the same number of iterations.

Upvotes: 0

Views: 563

Answers (1)

Zulan
Zulan

Reputation: 22670

Despite your attempt to synchronize with the barrier, you do have a race condition on working_threads that can easily lead to unequal amount of iterations:

thread 0                             | thread 1
...                                  | ...
while (working_threads > 0) [==true] | ...
    if (my_num == 1) [==true]        | ...
        working_threads = 0          | ...
                                     | while (working_threads > 0) [==false]
    [hangs waiting for barrier]      | [hangs trying to exit from parallel]

To fix your specific code, you would have to also add a barrier between the while-condition-check and working_threads = 0.

#pragma omp parallel
    {   
        int my_num=omp_get_thread_num()+1;
        int idle=false;
        while(working_threads>0) {
#pragma omp barrier      
            if(my_num==1)
                working_threads=0;
#pragma omp barrier                                                                                                                       
        }   
    }

Note that the code is not exactly the most idiomatic or elegant solution. Depending on your specific work-sharing problem, there may be a better approach. Also you must ensure that worker_threads is written only by a single thread - or use atomics when writing.

Upvotes: 1

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